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Do natural selection and survival of the fittest explain the divergent length of the recurrent laryngeal nerve...centimeters in the human, 15 feet in the giraffe?

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  • $\begingroup$ Are you suggesting the length of the nerve evolves independent to the length of the neck? That's not how development works at all $\endgroup$
    – Bryan Krause
    Dec 24 '19 at 16:19
  • $\begingroup$ I don’t understand what you’re asking. The divergence of all ancestral traits can be explained by evolution. I’m aware that the laryngeal nerve’s path under the arch of the aorta has been used as an argument against intelligent design; based on that and your comments on some of the answers, it appears to me that you’re just trying to start a discussion on intelligent design versus natural selection. For that reason, I have voted to close your question. Perhaps you could clarify why the divergence in length you mentioned is confusing to you. $\endgroup$
    – canadianer
    Dec 25 '19 at 7:07
  • $\begingroup$ I am expressing a fact. I find plausible that finch charateristics diverge on isolated islands. I don't find plausible that natural selection explains the lengthening of the r.l. nerve to 15 feet. Within the province of discussion of natural selection an appropriate scientific skepticism is not possible. Is it because the theory of the r. l. nerve by natural selection is "not falsifiable". This criterion of science seems to have been forgotten. I am entitled to doubt it and not be thought to entertain intelligent design. There nay be some agency in the genome which governs tandem association $\endgroup$
    – user56930
    Dec 25 '19 at 14:54
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    $\begingroup$ You are expected to show some attempt to research questions before you ask them, what research have you done. Clarity in what exactly you are asking would also be a desired improvement. $\endgroup$
    – John
    Dec 25 '19 at 19:09
  • $\begingroup$ I have research Darwinian explanation. I wrote a book about. If you would like I can send you the google drive link since I am trying to get feedback on it. I've read Popper. And I don't find that appropriate scientific skepticism is permitted in the field of natural selection. $\endgroup$
    – user56930
    Dec 27 '19 at 5:01
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This has a lot to do with how development works, there is no gene that makes the nerve this long. Instead, there are genes that tell the nerve where to grow and what path to take using other tissues as landmarks. At each step of the neck getting longer letting the nerve follow this path is more beneficial than the complex evolution of pathways necessary to change how it grows. At each step the added length of the nerve is a small detriment while a large number of simultaneous mutations would be needed to change the developmental pathways. Since these mutations are extremely unlikely to occur together and produce the desired results, they are instead a major/lethal detriment.

Based on fish it is likely that in the early common ancestor the nerve took a straight path. As necks evolved the heart moved down into the body and away from the gills the nerve originally connected to. Thus, the nerve is trapped by its own developmental pathway, it runs behind the gill arches which in tetrapods have become the aorta which is essential and the Ductus arteriosus which is developmentally vital. The nerves uses these as developmental landmarks for how it develops and grow in long before the neck develops. In an embryo (which has no neck) this is still a short straight trip, but as the embryo continues developing it has to grow longer and longer as the neck forms. Keep in mind the larynx and heart develop in very different location then where they end up in the body. Elongation of nerves as the tissue elongates does not need to be programed individually there is a general process that makes it happen in all nerves so no new mutations are needed. This is plasticity at work. The right and left nerve are actually different lengths because different gill arches are lost on the right and left side of mammals. Simply letting the nerve get longer is less detrimental than not having the nerve which are the two options. As I mentioned above rerouting the nerve is unlikely to happen due to the improbability of the co-occurrence of multiple mutations to completely reorganize developmental pathways. There is a cost (extra tissue and calories and some signal lag and even rare developmental errors in the most extreme cases), but since it controls most of the larynx not having the nerve would prevent vocalization or or even breathing properly. Given those two options it is fairly easy to see why evolution favors individuals that maintain this pattern.

Evolution is full of kluges and "good enough" solutions/compromises like this.

enter image description here

enter image description here

https://www.evolutionarymodel.com/apps/photos/photo?photoid=56959380

https://arxiv.org/pdf/1901.01560.pdf

https://web.duke.edu/anatomy/embryology/cardiovascular/cardiovascular.html

https://www.sciencedirect.com/science/article/abs/pii/S095943880500187X

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  • $\begingroup$ This is good answer, but I think it would be even better if you could provide some more references for particular statements (e.g. tissues being used as landmarks for nerve growth, number of genes involved in guiding this nerve, the movement of the heart during evolution & development, etc.) within the text and also indicate within the text where the figures are relevant. ——— I also took the liberty of editing in way I hope improves the clarity of your argument, but since this is out of my field it would be great if you corrected any errors I inadvertently introduced! 🙃 $\endgroup$
    – tyersome
    Dec 26 '19 at 19:38
  • $\begingroup$ The improbability of multiple simultaneous mutations is pretty basic I any link will just be to a textbook. but I can add some more references. $\endgroup$
    – John
    Dec 26 '19 at 20:40
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I'm not sure how to address this question so that we may arrive at a suitable answer, but I'm sure that the recurrent laryngeal nerve is an example of bad design. Evolution works from traits already possessed by the parents. Mammals evolved from fishes in our evolutionary past and fishes do not have a neck. As a result the recurrent laryngeal nerve of giraffes loops under the aortic arch instead of over the arch.

Offspring of giraffes that are born with a longer neck have more chances of surviving and passing on the trait because they have an easier time to access food sources in higher places and an easier time to watch out for predators. Simultaneously, by natural selection the nerve is gradually lengthened in tiny increments. As Richard Dawkins said in his book Outgrowing God, "evolution cannot go back to the drawing board. Evolution has no foresight.”1

References:

1: Dawkins, R. (2019). Outgrowing God: A Beginner's Guide. Random House.

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    $\begingroup$ The idea that varieties died out because their r.l. nerve was not long enough (for my poor brain) passes the bar of implausibilty. $\endgroup$
    – user56930
    Dec 24 '19 at 13:09
  • $\begingroup$ Also if I may add, there is the rare variation in which the left r.l. nerve is non-recurrent, direct from vagus n. to the innervated tissues. The tenets of natural selection would imply that this uncomplicated variation should have gained ascendancy $\endgroup$
    – user56930
    Dec 24 '19 at 13:17
  • $\begingroup$ Welcome to Biology.SE! This is a good start, but answers are much more likely to receive a favorable response if you include supporting references (primary literature is best). Without that support, your answer is indistinguishable from opinion. In addition, your answer could be interpreted as saying that the looping is giraffe specific. ——— Please take the tour and then consult the help pages for additional advice on How to Answer effectively on this site and then edit your answer accordingly. Thanks! 😊 $\endgroup$
    – tyersome
    Dec 24 '19 at 18:57
  • $\begingroup$ I also think that elaborating on how mammals (Is it really only mammals?) ended up with a recurrent laryngeal nerve would improve your answer. Finally, if you have a quotation from a book a complete reference to that work is best including a page number. (I've edited your post to improve the wording a bit and added the book reference, but don't have time to figure out the page.) $\endgroup$
    – tyersome
    Dec 24 '19 at 19:11
  • $\begingroup$ I wanted to delete one final comment and it looks like others were deleted. It was not intentional. If you want to pick up where we left off or restore them I would enjoy it. Thank you for your elaborate answer and comments. Natural Selection explains. Intelligent Design does not explain in the way that scientist love. $\endgroup$
    – user56930
    Dec 27 '19 at 5:40

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